(…)
In preferred IUPAC names, stereodescriptors are placed immediately at the front of the part of the name to which they relate. They are placed at the front of the complete name when related to the parent structure; they are cited in parentheses followed by a hyphen. When they relate to substituent groups, they are cited at the front of the corresponding prefix. They are preceded by a numerical or letter locant to describe the position of the stereogenic unit when such locants are present; general rules of numbering are applied (see P-14.4).
(…)
Accordingly, all stereodiscriptors have to be preceded by a locant.
In case of the PIN for cinnamaldehyde, the stereodiscriptor ‘E’ is preceded by the numerical locant ‘2’.
In general nomenclature, however, locants are usually omitted when there is no ambiguity.
Nevertheless, the current IUPAC recommendations are prescriptive about when omission of locants is permissible in PINs. Locants are omitted in PINs only in the exceptional cases that are explicitly mentioned in the IUPAC recommendations. None of these cases applies to the locant of the stereodiscriptor in (2E)-3-phenylprop-2-enal.
As a rule of thumb, if any locants are essential for defining the structure, then all locants must be cited in the PIN.
Note that the current recommendations given in the 2013 Blue Book incorporate a change from earlier recommendations given in the 1993 Guide. By way of comparison, the following examples are explicitly mentioned in the 1993 Recommendations as well as in the 2013 Recommendations:
(…) In preferred IUPAC names, stereodescriptors are placed immediately at the front of the part of the name to which they relate. They are placed at the front of the complete name when related to the parent structure; they are cited in parentheses followed by a hyphen. When they relate to substituent groups, they are cited at the front of the corresponding prefix. They are preceded by a numerical or letter locant to describe the position of the stereogenic unit when such locants are present; general rules of numbering are applied (see P-14.4). (…)
Accordingly, all stereodiscriptors have to be preceded by a locant.In case of the PIN for cinnamaldehyde, the stereodiscriptor ‘E’ is preceded by the numerical locant ‘2’.
In general nomenclature, however, locants are usually omitted when there is no ambiguity.
Nevertheless, the current IUPAC recommendations are prescriptive about when omission of locants is permissible in PINs. Locants are omitted in PINs only in the exceptional cases that are explicitly mentioned in the IUPAC recommendations. None of these cases applies to the locant of the stereodiscriptor in (2E)-3-phenylprop-2-enal.
As a rule of thumb, if any locants are essential for defining the structure, then all locants must be cited in the PIN.
Note that the current recommendations given in the 2013 Blue Book incorporate a change from earlier recommendations given in the 1993 Guide. By way of comparison, the following examples are explicitly mentioned in the 1993 Recommendations as well as in the 2013 Recommendations:
This is pretty surprising, but very informative, thanks. Im surprised that even something like a but-2-ene needs a number now, I wonder why they made that decision...More
The preferred IUPAC name (PIN) for cinnamaldehyde is (2E)-3-phenylprop-2-enal.
With regard to numbering of locants that describe the position of stereogenic units, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
Accordingly, all stereodiscriptors have to be preceded by a locant. In case of the PIN for cinnamaldehyde, the stereodiscriptor ‘E’ is preceded by the numerical locant ‘2’.
In general nomenclature, however, locants are usually omitted when there is no ambiguity.
Nevertheless, the current IUPAC recommendations are prescriptive about when omission of locants is permissible in PINs. Locants are omitted in PINs only in the exceptional cases that are explicitly mentioned in the IUPAC recommendations. None of these cases applies to the locant of the stereodiscriptor in (2E)-3-phenylprop-2-enal.
As a rule of thumb, if any locants are essential for defining the structure, then all locants must be cited in the PIN.
Note that the current recommendations given in the 2013 Blue Book incorporate a change from earlier recommendations given in the 1993 Guide. By way of comparison, the following examples are explicitly mentioned in the 1993 Recommendations as well as in the 2013 Recommendations:
(2E)-but-2-ene (PIN, 2013 Recommendations)
(E)-but-2-ene (1993 Recommendations)
(1Z)-1,2-dibromo-1-chloro-2-iodoethene (PIN, 2013 Recommendations)
(Z)-1,2-dibromo-1-chloro-2-iodoethene (1993 Recommendations)
(2Z)-2-methylbut-2-enoic acid (PIN, 2013 Recommendations)
(Z)-2-methylbut-2-enoic acid (1993 Recommendations)
(2E)-but-2-enedioic acid (PIN, 2013 Recommendations)
(E)-but-2-enedioic acid (1993 Recommendations)
The preferred IUPAC name (PIN) for cinnamaldehyde is (2E)-3-phenylprop-2-enal.
With regard to numbering of locants that describe the position of stereogenic units, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
Accordingly, all stereodiscriptors have to be preceded by a locant.In case of the PIN for cinnamaldehyde, the stereodiscriptor ‘E’ is preceded by the numerical locant ‘2’.
In general nomenclature, however, locants are usually omitted when there is no ambiguity.
Nevertheless, the current IUPAC recommendations are prescriptive about when omission of locants is permissible in PINs. Locants are omitted in PINs only in the exceptional cases that are explicitly mentioned in the IUPAC recommendations. None of these cases applies to the locant of the stereodiscriptor in (2E)-3-phenylprop-2-enal.
As a rule of thumb, if any locants are essential for defining the structure, then all locants must be cited in the PIN.
Note that the current recommendations given in the 2013 Blue Book incorporate a change from earlier recommendations given in the 1993 Guide. By way of comparison, the following examples are explicitly mentioned in the 1993 Recommendations as well as in the 2013 Recommendations:
(2E)-but-2-ene (PIN, 2013 Recommendations)
(E)-but-2-ene (1993 Recommendations)
(1Z)-1,2-dibromo-1-chloro-2-iodoethene (PIN, 2013 Recommendations)
(Z)-1,2-dibromo-1-chloro-2-iodoethene (1993 Recommendations)
(2Z)-2-methylbut-2-enoic acid (PIN, 2013 Recommendations)
(Z)-2-methylbut-2-enoic acid (1993 Recommendations)
(2E)-but-2-enedioic acid (PIN, 2013 Recommendations)
(E)-but-2-enedioic acid (1993 Recommendations)
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The 1993 IUPAC "Blue Book" is the easiest to find online thanks to ACDLabs. In this version, numbers are only used with stereochemical descriptors if there is more than one:
(E)-phenylprop-2-enal is the correct systematic name for cinnamaldehyde.
The most recent edition is the 2002 edition, which I do not have access to. The 2002 edition may contain a different recommendation.
The 1993 IUPAC "Blue Book" is the easiest to find online thanks to ACDLabs. In this version, numbers are only used with stereochemical descriptors if there is more than one:
(E)-phenylprop-2-enal is the correct systematic name for cinnamaldehyde.
The most recent edition is the 2002 edition, which I do not have access to. The 2002 edition may contain a different recommendation.
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